Co2 separation system

ABSTRACT

A CO2 separation system configured to separate CO2 from mixed gas containing CO2 includes a CO2 separator, a CO2 collector, and a pressure difference generator. The CO2 separator includes a separation membrane configured to separate the CO2 from the mixed gas, and a separation-membrane upstream chamber and a separation-membrane downstream chamber demarcated by the separation membrane. The CO2 separator is disposed to cause the mixed gas to flow into the separation-membrane upstream chamber. The pressure difference generator includes at least a negative pressure generator. The negative pressure generator is disposed on a gas path of the permeating gas that connects the separation-membrane downstream chamber and the CO2 collector.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Japanese Patent Application No. 2020-051589 filed on Mar. 23, 2020, incorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a CO₂ separation system.

2. Description of Related Art

Japanese Patent No. 6359544 (JP 6359544 B) discloses a carbon dioxide (CO₂) separation system configured such that CO₂ in exhaust gas discharged from an internal combustion engine is selectively separated by using a facilitated transport membrane and the separated CO₂ is condensed and stored.

SUMMARY

However, JP 6359544 B does not explicitly disclose how the separated CO₂ is collected eventually. Thus, there is a possibility that the separated CO₂ cannot be collected efficiently.

The present disclosure provides a CO₂ separation system that can efficiently collect CO₂ separated from mixed gas containing CO₂.

A CO₂ separation system according to one aspect of the present disclosure is configured to separate CO₂ from mixed gas containing the CO₂. The CO₂ separation system includes a CO₂ separator, a CO₂ collector, and a pressure difference generator. The CO₂ separator includes a separation membrane configured to separate the CO₂ from the mixed gas, and a separation-membrane upstream chamber and a separation-membrane downstream chamber demarcated by the separation membrane. The CO₂ separator is disposed to cause the mixed gas to flow into the separation-membrane upstream chamber. The CO₂ collector is configured such that permeating gas that contains the CO₂ is introduced into the CO₂ collector. The CO₂ collector is configured to collect the CO₂ in the permeating gas. The permeating gas is gas that has permeated the separation membrane and flowed out of the separation-membrane downstream chamber. The pressure difference generator configured to generate a pressure difference between the separation-membrane upstream chamber and the separation-membrane downstream chamber such that a pressure in the separation-membrane upstream chamber is higher than a pressure in the separation-membrane downstream chamber. The pressure difference generator includes at least a negative pressure generator. The negative pressure generator is disposed on a gas path of the permeating gas that connects the separation-membrane downstream chamber and the CO₂ collector.

In the CO₂ separation system according to the one aspect of the present disclosure, a CO₂ partial pressure difference between the separation-membrane upstream chamber and the separation-membrane downstream chamber, which serves as a permeation propulsion force when CO₂ permeates the separation membrane, is secured by the simple structure in which the negative pressure generator is provided on the gas path of the permeating gas. Therefore, the permeation of CO₂ can be promoted, and the permeating gas discharged from the negative pressure generator to have a pressure corresponding to an atmospheric pressure and have a high CO₂ concentration (that is, the permeating gas having a high CO₂ partial pressure) can be introduced into the CO₂ collector. Thus, the CO₂ collection amount of the CO₂ collector can be increased. Further, the pressure in the CO₂ collector is not a negative pressure. Therefore, it is possible to suppress leakage of CO₂ in the CO₂ collector to the outside via, for example, the gas path of the permeating gas. Thus, the CO₂ collector can efficiently collect CO₂ separated from exhaust gas (mixed gas) containing CO₂.

The CO₂ separation system according to the one aspect of the present disclosure may further include a dehumidifier configured to dehumidify the permeating gas. The dehumidifier and the negative pressure generator may be disposed on the gas path of the permeating gas in order of the dehumidifier and the negative pressure generator from an upstream side in a flow direction of the permeating gas. The CO₂ separation system according to the one aspect of the present disclosure may further include a catalyst device. The mixed gas may be exhaust gas discharged from an internal combustion engine mounted on a moving object or from a stationary internal combustion engine. The catalyst device may be disposed on an exhaust path of the internal combustion engine, and may be configured to remove a toxic substance in the exhaust gas. The CO₂ separator may be disposed on the exhaust path on a downstream side of the catalyst device in a flow direction of the exhaust gas. The CO₂ collector may be configured such that the permeating gas that contains the CO₂ and water vapor is introduced into the CO₂ collector. The permeating gas is gas that has permeated the separation membrane and flowed out of the separation-membrane downstream chamber. The dehumidifier and the negative pressure generator may be disposed on the gas path of the permeating gas that connects the separation-membrane downstream chamber and the CO₂ collector, in order of the dehumidifier and the negative pressure generator from an upstream side in a flow direction of the permeating gas.

The CO₂ separation system according to the one aspect of the present disclosure may further include an exhaust gas switching valve provided on the exhaust path between the catalyst device and the separation-membrane upstream chamber, and configured to switch whether to introduce the exhaust gas flowing out of the catalyst device into the separation-membrane upstream chamber or to discharge the exhaust gas flowing out of the catalyst device to outside air without introducing the exhaust gas into the separation-membrane upstream chamber. The CO₂ separation system according to the one aspect of the present disclosure may further include a heater. The dehumidifier may be configured to dehumidify the permeating gas by cooling the permeating gas through heat exchange with a heating medium and condensing moisture in the permeating gas. The heater may be configured to heat the CO₂ collector by using heat of the heating medium that is received through the heat exchange with the permeating gas.

In the CO₂ separation system according to the one aspect of the present disclosure, the heater may include a heat accumulator configured to accumulate the heat of the heating medium that is received through the heat exchange with the permeating gas, and a heat transfer unit configured to transfer the heat in the heat accumulator to the CO₂ collector to heat the CO₂ collector. In the CO₂ separation system according to the one aspect of the present disclosure, the mixed gas may be exhaust gas discharged from an internal combustion engine mounted on a moving object or from a stationary internal combustion engine. In the CO₂ separation system according to the one aspect of the present disclosure, the mixed gas may be air.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like signs denote like elements, and wherein:

FIG. 1 is a schematic structural diagram of a CO₂ separation system according to a first embodiment of the present disclosure;

FIG. 2 is a diagram illustrating a relationship between a CO₂ partial pressure in permeating gas and a CO₂ adsorption amount of a solid adsorbent per unit mass;

FIG. 3 is a flowchart for describing CO₂ separation control according to the first embodiment of the present disclosure;

FIG. 4 is a schematic structural diagram of a CO₂ separation system according to a second embodiment of the present disclosure;

FIG. 5 is a schematic structural diagram of a CO₂ separation system according to a third embodiment of the present disclosure;

FIG. 6 is a flowchart for describing CO₂ separation control according to the third embodiment of the present disclosure; and

FIG. 7 is a schematic structural diagram of a CO₂ separation system using a pressurization pump as a pressure difference generator in place of a depressurization pump.

DETAILED DESCRIPTION OF EMBODIMENTS

Referring to the drawings, detailed description is given of carbon dioxide (CO₂) separation systems according to embodiments of the present disclosure, which are configured to separate CO₂ from the atmosphere (air) or exhaust gas discharged from an internal combustion engine mounted on a moving object such as a vehicle or a ship, or a stationary internal combustion engine provided in a plant or the like. In the following description, the same components are represented by the same reference symbols.

First Embodiment

FIG. 1 is a schematic structural diagram of a CO₂ separation system 100 according to a first embodiment of the present disclosure.

The CO₂ separation system 100 according to this embodiment includes an internal combustion engine 10, a CO₂ separator 20, a dehumidifier 30, a pressure difference generator 40, a CO₂ collector 50, and an electronic control unit 200. The CO₂ separation system 100 separates CO₂ in exhaust gas discharged from the internal combustion engine 10.

The internal combustion engine 10 generates power for driving, for example, a vehicle by burning fuel in a combustion chamber formed in the internal combustion engine 10. The internal combustion engine 10 has an exhaust passage 11 through which exhaust gas generated in the combustion chamber (burned gas) is discharged into the atmosphere. The exhaust passage 11 is provided with a catalyst device 12 configured to remove toxic substances in the exhaust gas.

The CO₂ separator 20 includes a separation membrane 21, a separation-membrane upstream chamber 22, and a separation-membrane downstream chamber 23. The separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 are demarcated by the separation membrane 21. The CO₂ separator 20 is provided in the exhaust passage 11 on a downstream side of the catalyst device 12 in an exhaust gas flow direction to cause exhaust gas to flow into the separation-membrane upstream chamber 22.

The separation membrane 21 has a function of separating CO₂ from exhaust gas by causing CO₂ in the exhaust gas to permeate the separation membrane 21. The separation membrane 21 also has a function of separating, when exhaust gas contains water vapor, CO₂ together with water vapor from the exhaust gas by causing CO₂ and water vapor to permeate the separation membrane 21. Examples of the separation membrane 21 include a facilitated transport membrane, an organic polymer membrane, an inorganic material membrane, and a liquid membrane. The CO₂ and water vapor in the exhaust gas that flow into the separation-membrane upstream chamber 22 permeate the separation membrane 21 and flow into the separation-membrane downstream chamber 23 by using, as a permeation propulsion force, a pressure difference (partial pressure difference) between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 demarcated by the separation membrane 21. The CO₂ and water vapor that permeate the separation membrane 21 and flow into the separation-membrane downstream chamber 23 are hereinafter referred to as “permeating gas” as necessary.

The exhaust gas that flows into the separation-membrane upstream chamber 22 and has a CO₂ concentration reduced by separating CO₂ through the separation membrane 21 is discharged into the exhaust passage 11 on a downstream side of the separation-membrane upstream chamber 22 in the exhaust gas flow direction, and is eventually discharged to outside air. The permeating gas that flows into the separation-membrane upstream chamber 22, permeates the separation membrane 21, and flows into the separation-membrane downstream chamber 23 to have a high CO₂ concentration is discharged into a permeating gas passage 60 having one end coupled to the separation-membrane downstream chamber 23 and the other end coupled to the CO₂ collector 50.

The dehumidifier 30 removes moisture in gas. The dehumidifier 30 according to this embodiment includes a radiator 31, a coolant circulation passage 32, a moisture remover 33, and a circulation pump 34. The moisture remover 33 is provided in the permeating gas passage 60 so that moisture in the permeating gas can be removed at the moisture remover 33.

The radiator 31 has a coolant inlet 31 a, a core 31 b, and a coolant outlet 31 c. A high-temperature coolant introduced into the radiator 31 through the coolant inlet 31 a is cooled by exchanging heat with low-temperature gas such as air in the core 31 b and is discharged through the coolant outlet 31 c.

The coolant circulation passage 32 is a passage through which the coolant discharged from the coolant outlet 31 c of the radiator 31 is supplied to the moisture remover 33 to cool the permeating gas and then returned to the coolant inlet 31 a of the radiator 31, thereby circulating the coolant.

The moisture remover 33 is provided in the permeating gas passage 60, and is configured to remove moisture in the permeating gas such that heat is exchanged between relatively high-temperature permeating gas flowing into the moisture remover 33 and a relatively low-temperature coolant supplied to the moisture remover 33 through the coolant circulation passage 32 to cool the permeating gas and condensed water produced by cooling the permeating gas is drained to the outside.

The circulation pump 34 is provided in the coolant circulation passage 32. The circulation pump 34 circulates the coolant in the coolant circulation passage 32 by being driven when dehumidifying (cooling) the permeating gas at the moisture remover 33.

In this embodiment, the permeating gas that permeates the separation membrane 21 is dehumidified instead of dehumidifying the exhaust gas before CO₂ and water vapor are separated by the separation membrane 21. The reason is as follows.

In general, the separation membrane 21 has such a characteristic that the membrane permeation rate of water vapor is higher than the membrane permeation rate of CO₂. When the exhaust gas contains water vapor, CO₂ in the separation-membrane downstream chamber 23 is diluted by the water vapor having the higher membrane permeation rate, and the mole fraction of CO₂ decreases. As a result, a CO₂ partial pressure difference between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23, which serves as the permeation propulsion force of CO₂, increases to promote permeation of CO₂. Thus, the separation of CO₂ in the CO₂ separator 20 can be promoted. That is, the separation of CO₂ in the CO₂ separator 20 can be promoted when the exhaust gas containing water vapor is introduced into the separation-membrane upstream chamber 22. For the reason described above, the permeating gas that permeates the separation membrane 21 is dehumidified in this embodiment instead of dehumidifying the exhaust gas before CO₂ and water vapor are separated by the separation membrane 21.

In this embodiment, the dehumidifier 30 removes moisture in the permeating gas by draining condensed water produced by cooling the permeating gas. The structure of the dehumidifier 30 is not limited to this structure. For example, the dehumidifier 30 may remove moisture in the permeating gas by adsorbing the moisture in the permeating gas on a desiccant having a good water absorbing property.

The pressure difference generator 40 generates a pressure difference between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 such that a pressure in the separation-membrane upstream chamber 22 is higher than a pressure in the separation-membrane downstream chamber 23. In this embodiment, at least a depressurization pump is used as the pressure difference generator 40. The depressurization pump is provided in the permeating gas passage 60 on a downstream side of the moisture remover 33 in the gas flow direction to reduce the pressure in the separation-membrane downstream chamber 23 to a pressure lower than an atmospheric pressure (negative pressure). In this manner, the pressure difference is generated between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 such that the pressure in the separation-membrane upstream chamber 22 atmospheric pressure) is higher than the pressure in the separation-membrane downstream chamber 23 (=negative pressure).

By providing the depressurization pump serving as the pressure difference generator 40 in the permeating gas passage 60 on the downstream side of the dehumidifier 30 in the gas flow direction, gas to be sucked into the depressurization pump is dehumidified by the dehumidifier 30. Therefore, it is possible to suppress a breakdown or deterioration of the depressurization pump due to water vapor (moisture) in the sucked gas.

As described above, the other end of the permeating gas passage 60 is coupled to the CO₂ collector 50. Permeating gas dehumidified by the dehumidifier 30 and discharged from the depressurization pump to have a pressure corresponding to the atmospheric pressure is introduced into the CO₂ collector 50. The CO₂ collector 50 includes a solid adsorbent such as zeolite or activated carbon, and collects CO₂ in the permeating gas introduced into the CO₂ collector 50 via the permeating gas passage 60 by adsorbing CO₂ on the solid adsorbent.

In this embodiment, a CO₂ sensor 201 is provided in the permeating gas passage 60 between the pressure difference generator 40 and the CO₂ collector 50 to estimate an adsorption amount of CO₂ adsorbed on the solid adsorbent of the CO₂ collector 50. The CO₂ sensor 201 detects a flow rate of the permeating gas to be introduced into the CO₂ collector 50, and a CO₂ concentration in the permeating gas.

The permeating gas having a CO₂ concentration reduced by adsorbing CO₂ on the solid adsorbent is discharged to outside air from the CO₂ collector 50. The CO₂ collected by the CO₂ collector 50, that is, the CO₂ adsorbed on the solid adsorbent can be ejected to the outside through a CO₂ ejection port (not illustrated) formed at an arbitrary position on the CO₂ collector 50 by heating the CO₂ collector 50 and furthermore the solid adsorbent to desorb, from the solid adsorbent, the CO₂ adsorbed on the solid adsorbent.

When the permeating gas contains water vapor, the adsorbability of the solid adsorbent such as zeolite tends to decrease. In this embodiment, the decrease in the adsorbability of the solid adsorbent can be suppressed by introducing the dehumidified permeating gas into the CO₂ collector 50.

FIG. 2 is a diagram illustrating a relationship between a CO₂ partial pressure in gas and a CO₂ adsorption amount of a solid adsorbent per unit mass. As illustrated in

FIG. 2, the CO₂ adsorption amount of the solid adsorbent per unit mass tends to increase as the CO₂ partial pressure in the gas increases. That is, the CO₂ adsorption amount of the solid adsorbent per unit mass can be increased as a CO₂ partial pressure in the permeating gas is increased compared to a CO₂ partial pressure in the exhaust gas.

In this embodiment, the dehumidifier 30 and the pressure difference generator 40 are arranged in the permeating gas passage 60 in this order from an upstream side in the gas flow direction. Therefore, permeating gas dehumidified by the dehumidifier 30 and discharged from the depressurization pump serving as the pressure difference generator 40 to have a pressure corresponding to the atmospheric pressure and have a high CO₂ concentration (that is, permeating gas having a high CO₂ partial pressure) can be introduced into the CO₂ collector 50. Thus, the CO₂ adsorption amount of the CO₂ collector 50 can be increased.

Referring back to FIG. 1, the electronic control unit 200 is a microcomputer including a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), an input port, and an output port, which are connected together via a bidirectional bus. The electronic control unit 200 is configured to perform CO₂ separation control for separating CO₂ in exhaust gas and collecting the CO₂ into the CO₂ collector 50 during operation of the internal combustion engine 10. Therefore, signals are input to the electronic control unit 200 from various sensors such as the CO₂ sensor 201 necessary in the CO₂ separation control.

FIG. 3 is a flowchart for describing the CO₂ separation control according to this embodiment to be performed by the electronic control unit 200.

In Step S1, the electronic control unit 200 determines whether the internal combustion engine 10 is operating, that is, whether exhaust gas is discharged from the internal combustion engine 10 to the exhaust passage 11. When the internal combustion engine 10 is operating, the electronic control unit 200 proceeds to a process of Step S2. When the internal combustion engine 10 is not operating, the electronic control unit 200 terminates this process.

In Step S2, the electronic control unit 200 determines whether the adsorption amount of CO₂ adsorbed on the solid adsorbent of the CO₂ collector 50 is smaller than a predetermined upper limit adsorption amount. When a CO₂ adsorption amount estimated based on a detection value from the CO₂ sensor 201 is smaller than the upper limit adsorption amount, the electronic control unit 200 proceeds to a process of Step S3. When the CO₂ adsorption amount is equal to or larger than the upper limit adsorption amount, the electronic control unit 200 terminates this process because the CO₂ collector 50 cannot collect CO₂ over the upper limit adsorption amount.

In Step S3, the electronic control unit 200 drives the pressure difference generator 40 to generate a pressure difference between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 such that the pressure in the separation-membrane upstream chamber 22 is higher than the pressure in the separation-membrane downstream chamber 23. That is, a CO₂ partial pressure difference serving as the permeation propulsion force when CO₂ permeates the separation membrane 21 is generated between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23. The electronic control unit 200 also drives the circulation pump 34 to dehumidify permeating gas at the moisture remover 33.

According to this embodiment, the CO₂ separation system 100 configured to separate CO₂ from exhaust gas (mixed gas) containing CO₂ includes the CO₂ separator 20, the pressure difference generator 40, and the CO₂ collector 50. The CO₂ separator 20 includes the separation membrane 21 having the function of separating CO₂ from the exhaust gas, and the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 demarcated by the separation membrane 21. The CO₂ separator 20 is arranged to cause the exhaust gas to flow into the separation-membrane upstream chamber 22. The pressure difference generator 40 generates a pressure difference between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 such that the pressure in the separation-membrane upstream chamber 22 is higher than the pressure in the separation-membrane downstream chamber 23. Permeating gas that contains CO₂ is introduced into the CO₂ collector 50. The CO₂ collector 50 collects the CO₂ in the permeating gas. The permeating gas is gas that has permeated the separation membrane 21 and flowed out of the separation-membrane downstream chamber 23. The pressure difference generator 40 includes at least the depressurization pump that is an example of a negative pressure generator. The depressurization pump is arranged in the permeating gas passage 60 connecting the separation-membrane downstream chamber 23 and the CO₂ collector 50 (on a gas path of the permeating gas).

Accordingly, the CO₂ partial pressure difference between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23, which serves as the permeation propulsion force when CO₂ permeates the separation membrane 21, is secured by the simple structure in which the depressurization pump is provided in the permeating gas passage 60. Therefore, the permeation of CO₂ can be promoted, and the permeating gas discharged from the depressurization pump to have a pressure corresponding to the atmospheric pressure and have a high CO₂ concentration (that is, the permeating gas having a high CO₂ partial pressure) can be introduced into the CO₂ collector 50. Thus, the CO₂ adsorption amount of the CO₂ collector can be increased. Further, the pressure in the CO₂ collector 50 is not a negative pressure. Therefore, it is possible to suppress leakage of CO₂ in the CO₂ collector 50 to the outside via the permeating gas passage 60 or the like. Thus, the CO₂ collector 50 can efficiently collect CO₂ separated from the exhaust gas (mixed gas) containing CO₂.

The CO₂ separation system 100 according to this embodiment further includes the dehumidifier 30 configured to dehumidify the permeating gas. The dehumidifier 30 and the depressurization pump serving as the pressure difference generator 40 are arranged in the permeating gas passage 60 (on the gas path of the permeating gas) in the order of the dehumidifier 30 and the depressurization pump from the upstream side in the gas flow direction.

The exhaust gas containing water vapor is introduced into the separation-membrane upstream chamber 22, and the permeating gas that permeates the separation membrane 21 is dehumidified instead of dehumidifying the exhaust gas before CO₂ and water vapor are separated by the separation membrane 21. Thus, the separation of CO₂ in the CO₂ separator 20 can be promoted as described above.

The depressurization pump serving as the pressure difference generator 40 is provided in the permeating gas passage 60 on the downstream side of the dehumidifier 30 in the gas flow direction. Thus, it is possible to suppress a breakdown or deterioration of the depressurization pump due to water vapor (moisture) in the gas.

The dehumidified permeating gas is introduced into the CO₂ collector 50. Thus, a decrease in the adsorbability of zeolite can be suppressed.

Second Embodiment

Next, a second embodiment of the present disclosure is described. This embodiment differs from the first embodiment in that the CO₂ collector 50 can be heated by using exhaust heat from the dehumidifier 30. The difference is mainly described below.

FIG. 4 is a schematic structural diagram of a CO₂ separation system 100 according to this embodiment.

To eject CO₂ collected by the CO₂ collector 50 to the outside from the CO₂ collector 50, it is necessary, as described above, to heat the CO₂ collector 50 and furthermore the solid adsorbent to desorb, from the solid adsorbent the CO₂ adsorbed on the solid adsorbent. When the CO₂ collector 50 is heated by using, for example, an electric heater, electric energy is newly needed to eject CO₂. Thus, energy consumption increases.

The CO₂ separation system 100 according to this embodiment includes a heater 70 configured to heat the CO₂ collector 50 by using exhaust heat from the dehumidifier 30 to minimize the energy consumption when ejecting CO₂.

The heater 70 is configured to heat the CO₂ collector 50 by using exhaust heat from the dehumidifier 30. The heater 70 includes a three-way valve 71, a coolant branch passage 72, a heat accumulator 73, and a heat transfer unit 74.

The three-way valve 71 is provided in the coolant circulation passage 32 through which a relatively high-temperature coolant flows after exchanging heat with high-temperature permeating gas at the moisture remover 33. In this embodiment, the three-way valve 71 can be switched to choose whether to directly introduce, into the radiator 31, the relatively high-temperature coolant after exchanging heat with the high-temperature permeating gas at the moisture remover 33 or to introduce the coolant into the radiator 31 via the heat accumulator 73 along the coolant branch passage 72. The three-way valve 71 is switched by the electronic control unit 200.

The heat accumulator 73 is provided in the coolant branch passage 72, and is configured to accumulate heat energy received from the relatively high-temperature coolant flowing through the coolant branch passage 72.

The heat transfer unit 74 is configured to transfer the heat in the heat accumulator 73 to the CO₂ collector 50 to heat the CO₂ collector 50 when ejecting CO₂ from the CO₂ collector 50. Examples of the heat transfer unit 74 include a heat pipe.

According to this embodiment, the dehumidifier 30 is configured to dehumidify the permeating gas by cooling the permeating gas through heat exchange with the coolant (heating medium) and condensing moisture in the permeating gas. The CO₂ separation system 100 further includes the heater 70 configured to heat the CO₂ collector 50 by using heat of the coolant (heating medium) that is received through the heat exchange with the permeating gas. Specifically, the heater 70 includes the heat accumulator 73 configured to accumulate the heat of the coolant (heating medium) that is received through the heat exchange with the permeating gas, and the heat transfer unit 74 configured to transfer the heat in the heat accumulator 73 to the CO₂ collector to heat the CO₂ collector.

Accordingly, when ejecting CO₂ collected by the CO₂ collector 50 to the outside from the CO₂ collector 50, the CO₂ collector 50 can be heated by using the exhaust heat from the dehumidifier 30. Thus, the energy consumption can be reduced when ejecting CO₂.

Third Embodiment

Next, a third embodiment of the present disclosure is described. This embodiment differs from the first embodiment in that switching can be made to choose whether to supply exhaust gas to the CO₂ separator 20. The difference is mainly described below.

FIG. 5 is a schematic structural diagram of a CO₂ separation system 100 according to this embodiment.

As illustrated in FIG. 5, the CO₂ separation system 100 according to this embodiment is configured such that exhaust gas discharged from the catalyst device 12 is discharged to outside air by bypassing the separation-membrane upstream chamber 22 of the CO₂ separator 20 as necessary without being supplied to the separation-membrane upstream chamber 22.

Specifically, the CO₂ separation system 100 according to this embodiment includes an exhaust gas switching valve 13 and an exhaust gas bypass passage 14. The exhaust gas switching valve 13 is provided on a downstream side of the catalyst device 12 in the exhaust gas flow direction and on an upstream side of the separation-membrane upstream chamber 22 of the CO₂ separator 20 in the exhaust gas flow direction. The exhaust gas discharged from the catalyst device 12 is discharged to the outside air by bypassing the separation-membrane upstream chamber 22 through the exhaust gas bypass passage 14.

The exhaust gas switching valve 13 is a three-way valve configured to be switchable to choose whether to cause the exhaust gas discharged from the catalyst device 12 to directly flow into the exhaust passage 11 on a downstream side of the exhaust gas switching valve 13 in the exhaust gas flow direction or to cause the exhaust gas to flow into the exhaust gas bypass passage 14. The switching is controlled by the electronic control unit 200.

For example, when the temperature of the exhaust gas discharged from the catalyst device 12 is higher than a heat resistant temperature of the separation membrane 21, the exhaust gas discharged from the catalyst device 12 can be caused to flow into the exhaust gas bypass passage 14 by the exhaust gas switching valve 13. Thus, deterioration of the separation membrane 21 can be suppressed.

FIG. 6 is a flowchart for describing CO₂ separation control according to this embodiment to be performed by the electronic control unit 200. In FIG. 6, processes similar to those in the first embodiment are performed in Step S1 to Step S3. Therefore, their description is omitted.

In Step S11, the electronic control unit 200 determines whether the temperature of the exhaust gas discharged from the catalyst device 12 is lower than a predetermined separation-membrane heat resistant temperature (for example, 180° C.). When the temperature of the exhaust gas discharged from the catalyst device 12 is lower than the separation-membrane heat resistant temperature, the electronic control unit 200 proceeds to the process of Step S2. When the temperature of the exhaust gas discharged from the catalyst device 12 is equal to or higher than the separation-membrane heat resistant temperature, the electronic control unit 200 proceeds to the process of Step S13.

In Step S12, the electronic control unit 200 controls the exhaust gas switching valve 13 to cause the exhaust gas discharged from the catalyst device 12 to flow into the separation-membrane upstream chamber 22.

In Step S13, the electronic control unit 200 controls the exhaust gas switching valve 13 to cause the exhaust gas discharged from the catalyst device 12 to flow into the exhaust gas bypass passage 14.

In the CO₂ separation system 100 according to this embodiment, switching can be made to choose whether to cause the exhaust gas discharged from the catalyst device 12 to flow into the separation-membrane upstream chamber 22 of the CO₂ separator 20 or to cause the exhaust gas to flow into the exhaust gas bypass passage 14. Therefore, when the temperature of the exhaust gas is higher than the heat resistant temperature of the separation membrane 21 as described above, the exhaust gas discharged from the catalyst device 12 can be caused to flow into the exhaust gas bypass passage 14 by the exhaust gas switching valve 13. Thus, deterioration of the separation membrane 21 can be suppressed. When the exhaust gas need not flow into the separation-membrane upstream chamber 22, the exhaust gas can be caused to flow into the exhaust gas bypass passage 14 and discharged to the outside air. Thus, it is possible to suppress soiling of the separation membrane 21 due to the exhaust gas.

Although the embodiments of the present disclosure are described above, the embodiments are only illustrative of a part of application examples of the present disclosure, but are not intended to limit the technical scope of the present disclosure to the specific structures of the embodiments.

For example, the depressurization pump is used as the pressure difference generator 40 in the embodiments described above. A pressurization pump may be used together with or in place of the depressurization pump.

FIG. 7 is a schematic structural diagram of a CO₂ separation system 100 using the pressurization pump as the pressure difference generator 40 in place of the depressurization pump.

Similarly to the third embodiment, the CO₂ separation system 100 illustrated in FIG. 7 is configured such that exhaust gas discharged from the catalyst device 12 is discharged to outside air by bypassing the separation-membrane upstream chamber 22 of the CO₂ separator 20 as necessary without being supplied to the separation-membrane upstream chamber 22.

In the CO₂ separation system 100 illustrated in FIG. 7, the pressurization pump serving as the pressure difference generator 40 is provided in the exhaust passage 11 between the exhaust gas switching valve 13 and the separation-membrane upstream chamber 22 of the CO₂ separator 20. By using the pressurization pump as the pressure difference generator 40 and providing the pressurization pump in the exhaust passage 11 on an upstream side of the separation-membrane upstream chamber 22 in the exhaust gas flow direction, the pressure in the separation-membrane upstream chamber 22 is made higher than the atmospheric pressure. In this manner, the pressure difference may be generated between the separation-membrane upstream chamber 22 and the separation-membrane downstream chamber 23 such that the pressure in the separation-membrane upstream chamber 22 is higher than the pressure in the separation-membrane downstream chamber 23.

In the embodiments described above, the exhaust gas discharged from the internal combustion engine 10 is introduced into the separation-membrane upstream chamber 22. In place of the exhaust gas, the atmosphere (air) may be introduced into the separation-membrane upstream chamber 22 and CO₂ in the atmosphere may be separated by the CO₂ separator 20. In this case, for example, the pressurization pump is used as the pressure difference generator 40 as illustrated in FIG. 7 to force the air into the separation-membrane upstream chamber 22. Thus, the CO₂ in the atmosphere can efficiently be introduced into and separated by the CO₂ separator 20.

In the embodiments described above, the CO₂ collector 50 collects CO₂ by so-called physical adsorption, in which CO₂ is adsorbed on the solid adsorbent by bringing gas containing CO₂ (permeating gas) into contact with the solid adsorbent, and the CO₂ is collected by desorbing the CO₂ from the solid adsorbent through heating (or depressurization). The method for collecting CO₂ is not particularly limited, and CO₂ may be collected by various methods such as physical absorption, chemical absorption, and cryogenic separation.

In the physical absorption, gas containing CO₂ is brought into contact with an absorbent solution in which CO₂ can be dissolved (for example, methanol or ethanol), and the CO₂ is physically absorbed into the absorbent solution under high pressure and low temperature. The CO₂ is collected from the absorbent solution through heating (or depressurization).

In the chemical absorption, gas containing CO₂ is brought into contact with an absorbent solution in which CO₂ can be dissolved selectively (for example, amine), and the CO₂ is absorbed into the absorbent solution through chemical reaction. The CO₂ is collected by dissociating the CO₂ from the absorbent solution through heating.

In the cryogenic separation, gas containing CO₂ is compressed and cooled to liquefy the CO₂. The CO₂ is collected by selectively distilling the liquefied CO₂. 

What is claimed is:
 1. A CO₂ separation system configured to separate CO₂ from mixed gas containing the CO₂, the CO₂ separation system comprising: a CO₂ separator including a separation membrane configured to separate the CO₂ from the mixed gas, and a separation-membrane upstream chamber and a separation-membrane downstream chamber demarcated by the separation membrane, the CO₂ separator being disposed to cause the mixed gas to flow into the separation-membrane upstream chamber; a CO₂ collector configured such that permeating gas that contains the CO₂ is introduced into the CO₂ collector, the CO₂ collector being configured to collect the CO₂ in the permeating gas, and the permeating gas being gas that has permeated the separation membrane and flowed out of the separation-membrane downstream chamber; and a pressure difference generator configured to generate a pressure difference between the separation-membrane upstream chamber and the separation-membrane downstream chamber such that a pressure in the separation-membrane upstream chamber is higher than a pressure in the separation-membrane downstream chamber, wherein: the pressure difference generator includes at least a negative pressure generator; and the negative pressure generator is disposed on a gas path of the permeating gas that connects the separation-membrane downstream chamber and the CO₂ collector.
 2. The CO₂ separation system according to claim 1, further comprising a dehumidifier configured to dehumidify the permeating gas, wherein the dehumidifier and the negative pressure generator are disposed on the gas path of the permeating gas in order of the dehumidifier and the negative pressure generator from an upstream side in a flow direction of the permeating gas.
 3. The CO₂ separation system according to claim 2, further comprising a catalyst device, wherein: the mixed gas is exhaust gas discharged from an internal combustion engine mounted on a moving object or from a stationary internal combustion engine; the catalyst device is disposed on an exhaust path of the internal combustion engine, and is configured to remove a toxic substance in the exhaust gas; the CO₂ separator is disposed on the exhaust path on a downstream side of the catalyst device in a flow direction of the exhaust gas; the CO₂ collector is configured such that the permeating gas that contains the CO₂ and water vapor is introduced into the CO₂ collector, and the permeating gas is gas that has permeated the separation membrane and flowed out of the separation-membrane downstream chamber; and the dehumidifier and the negative pressure generator are disposed on the gas path of the permeating gas that connects the separation-membrane downstream chamber and the CO₂ collector, in order of the dehumidifier and the negative pressure generator from an upstream side in a flow direction of the permeating gas.
 4. The CO₂ separation system according to claim 3, further comprising an exhaust gas switching valve provided on the exhaust path between the catalyst device and the separation-membrane upstream chamber, and configured to switch whether to introduce the exhaust gas flowing out of the catalyst device into the separation-membrane upstream chamber or to discharge the exhaust gas flowing out of the catalyst device to outside air without introducing the exhaust gas into the separation-membrane upstream chamber.
 5. The CO₂ separation system according to claim 2, further comprising a heater, wherein: the dehumidifier is configured to dehumidify the permeating gas by cooling the permeating gas through heat exchange with a heating medium and condensing moisture in the permeating gas; and the heater is configured to heat the CO₂ collector by using heat of the heating medium that is received through the heat exchange with the permeating gas.
 6. The CO₂ separation system according to claim 5, wherein the heater includes: a heat accumulator configured to accumulate the heat of the heating medium that is received through the heat exchange with the permeating gas; and a heat transfer unit configured to transfer the heat in the heat accumulator to the CO₂ collector to heat the CO₂ collector.
 7. The CO₂ separation system according to claim 1, wherein the mixed gas is exhaust gas discharged from an internal combustion engine mounted on a moving object or from a stationary internal combustion engine.
 8. The CO₂ separation system according to claim 1, wherein the mixed gas is air. 